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150                 PA THOGENIC BA CTERIA.

chromate of potassium 2,. 2.5, sulphate of sodium i,
water 100), where it is hardened. When quite firm it
is washed in water, passed through alcohols ascending
in strength from 50 to 100 per cent., imbedded in eel-
loidin, cut wet, and stained like a section of tissue.

A ready method of doing this has been suggested by
Winkler,1 who bores a hole in a block of paraffin with
the smallest-size cork-borer, soaks the block in bichlorid
solution for an hour, pours liquid gelatin into the cavity,
allows it to solidify, inoculates it by the customary punc-
ture of the platinum wire, allows it to develop sufficiently,
and when ready cuts the sections under alcohol, subse-
quently staining them with much-diluted carbol-fuchsin.

Very pretty museum specimens of plate- and puncture-
cultures in gelatin can be made by simultaneously killing
the micro-organisms and permanently fixing the gelatin
with formalin, which can either be sprayed upon the
gelatin or applied in dilute solution. As gelatin fixed
in formalin cannot subsequently be liquefied, such prep-
arations will last indefinitely.

The growths which occur upon agar-agar are in many
ways less characteristic than those in gelatin, but as this
medium does not liquefy except at a high temperature
(100 C.), it has that great advantage over gelatin. The
colorless or almost colorless condition of the preparation
also aids in the detection of such chromogenesis as may
be the result of the micro-organismal growth.

Sometimes the growth is colored, sometimes not; some-
times the production of a soluble pigment colors the
agar-agar as well as the growth; sometimes the growth
is one color and the agar-agar another. Sometimes the
growth is filamentous, sometimes a smooth, shining band.
Occasionally the bacterium does not grow upon agar-agar
unless glycerin be added (tubercle bacillus); sometimes
it will not grow even then (gonococcus).

Still less characteristic are the growths upon potato.
Most bacteria produce rather smooth, shining, irregu-

1 Fortschritte der Medicin, Bd. xi., 1893, No. 22.